This invention relates generally to cutting apparatus and, more particularly, to boring apparatus of the type in which a cutting tooth is secured to a selectively rotatable shaft which is telescoped eccentrically into a power-rotated boring bar. By turning the eccentric shaft through a partial revolution within the bar, the cutting tooth may be adjusted from an inactive retracted position to an active cutting position, the tooth projecting outwardly beyond the periphery of the bar when moved to its cutting position. Boring apparatus of this type frequently is used to perform a back boring operation wherein the bar is thrust through a previously drilled bore while the cutting tooth is retracted and then is backed reversely out of the bore while the tooth is in its outwardly projected cutting position so as to remove material from the wall of the bore.